The present invention relates to a sun visor shaft and in particular to a sun visor shaft having an axial portion of the shaft formed by a plastic material with self-lubricating properties in contact with the sun visor detent spring.
For years the applicant has been working on the subject of a sun visor shaft with a greased bearing, but where the final objective of "no grease" has not yet been achieved until now. Greasing is required today in order to keep the annoying effect of squeaking (axle/spring are running dry) from occurring and to prevent the increase of the tilting torque in the case of extreme loads (tilting, heat, cold).
In the past, the most diverse experiments in the form of tests of the most varied types of grease, different surface qualities of the detent springs (by means of, for example, zinc-containing coating materials), as well as of a higher quality of finish on the surface of the shaft, were conducted. The combinations of tests did not result in a design that was free of grease, but merely in the discovery of improved types of grease in combination with an improvement of the surfaces of the spring and the shaft.
First of all, with respect to "greasing", it must be said that this constitutes a so-called "unclean" method of treatment, which is carried out with some reluctance. Furthermore, the application of grease does not constitute a process of satisfactory reliability, because in spite of the use of pneumatically or hydraulically operating greasing systems, air bubbles can occur in the grease and it can happen that no grease reaches the bearing or the detent spring. Furthermore, an important defective aspect has to be seen in the fact that grease residues can be present on the body of the sun visor, which, during the subsequent process, can cause invisible grease tracks to be present on the foil, which have later resulted in noticeable spots on the visible side of the sun visor body and have led to many complaints. In the case of electrical sun visors (a cable guided through a hollow shaft), the cables are coated with grease; this is bothersome and leads to the same defect as described earlier.
In the case of sun visors with electrical zones of contact at the shaft and the bearing, a not properly targeted or an excessive application of grease results in electrical contact problems, which has also led to a considerable number of complaints.
For example, in the case of visor bodies made of EPP, it has been shown during/after climate tests, that the grease migrates (through the material of the body), and thus comes in contact with the foil. For certain types of foils this leads to the formation of spots and distortions.
In summary, the above mentioned factors show that "greasing" constitutes a risk even though process and control methods are utilized.
It is an object of this invention to provide a solution, which makes it possible to dispense with the required greasing process in order to eliminate the above defects. Furthermore, the invention has the purpose of achieving distinction by virtue of less wear against steel (detent spring), less friction and slip-stick effect, lower moisture absorption, lower density (weight/cost savings), elimination of an additional lubrication process as well as the elimination of the risk of contamination by lubricants.
Dry friction is the main reason for wear. When materials come into contact with each other, the surfaces cling to each other, while fine particles are worn away from one of the two surfaces. Molten polymer, large grooves and furrows are indications of a high degree of wear and of the fact that the limit of pressure/velocity of the material has been exceeded.
In the case of, for example, a metal-thermoplastic pairing, the friction manifests itself by adhesion and deformation of the thermoplastic material. This dual effect has the result that the dynamic coefficient of friction is greater than the static coefficient of friction.
Different combinations or different surface properties of the frictional partners may influence the wear-frictional properties of thermoplastic materials. Also having an effect is the surface roughness of the metal as well as that of the thermoplastic material. In general the wear behavior depends on the surface hardness.
This invention relies on the use of thermoplastics with slip additives, which have better wear and frictional properties than unmodified thermoplastics. This makes the thermoplastic material ideal for the kind of use where self-lubrication, reliability, reduction of noise and wear are important.
For the sun visor shaft according to the invention, thermoplastics, in particular, which have been enriched with PTFE (polytetrafluoroethylene), should be considered, because these materials have a significantly lower wear factor and the lowest coefficient of friction. But also thermoplastics with silicon are advantageous, because they improve the wear properties and the frictional behavior to a large extent. Graphite powder (a highly temperature resistant solid with low friction), or molybdenum disulfide also could be used as an additional slip additive, while the latter, which is a solid, when added, for example to polyamide, reduces the wear factor. Furthermore, molybdenum disulfide transfers the lubricating property and provides a fine grain structure to the molded part.
The plastic material according to the invention, having self-lubricating properties, can also be enriched with reinforcing materials, such as glass fiber components, polymers or carbon fibers, aramide fibers or the like. Of all the reinforcing materials, aramide fibers offer the best lubricating effect and can reduce the wear factor of thermoplastics drastically (e.g. in combination with PTFE, the wear factor as well as the coefficient of friction are clearly lower).
Examples of embodiments of the invention will be explained below in more detail by reference to the drawings.